Contemporary magnetic disk drives typically include a rotating rigid storage disk and a head positioner for positioning a sensitive data transducer at different radial locations relative to the axis of rotation of the disk. Each location followed by the transducer defines a concentric data storage track on each recording surface of the disk. The read/write transducer, which may be of a single or dual element design, is typically deposited upon a ceramic slider structure having a highly polished air bearing surface for supporting the to transducer at a very small distance away from the surface of the moving disk storage medium.
Single write/read inductive element designs have typically required two wire connections, while dual element designs having separate reader and writer elements require four wire connections. Magnetoresistive (MR) heads in particular generally require four connection paths. The combination of an air bearing slider and a read/write transducer is also known as a read/write head or a recording head.
A head select, write driver, read preamplifier integrated circuit is physically located as close as practical to the read element to reduce pickup of unwanted electromagnetic noise. In the past, single element read/write electromagnetic transducers have been coupled to the read preamplifier via minute twisted wires dressed along the head positioner structure. More recently, printed circuit techniques employing linear parallel traces have been adopted to provide more control and uniformity in the connection path between the head and the preamplifier. These techniques have employed a trace suspension array (TSA) which provides a dual function of connecting the head and suspending the head over the disk by forming a gimbal enabling the head to pitch and roll to conform with the disk surface during flight. The TSA has then connected to a flexible trace interconnect (FIC) which then extends the signal paths from the head suspension to a printed circuit, called a "flex circuit" typically mounted on the head positioner structure.
Disk drive performance may be degraded by a particular operating or use environment. The disk drive designer and manufacturer who does not also assemble and integrate the disk drive into a completed computing system has virtually no control over the ultimate operating environment, and must take preventative and sometimes remedial steps to reduce electromagnetic noise interference. It has been learned through careful measurements that degradation of drive performance from EMI comes mainly from electromagnetic noise radiated externally of the drive which is picked up by an external cable connecting the drive to the computing system. The cable acts as an antenna and couples the externally sourced EMI into the drive and particularly into the sensitive front-end head signal preamplification circuitry of the drive. In particular, placement of the disk drive in close proximity to a magnetically noisy appliance, such as a compact disk (CD) player, within a computer housing, and/or the close proximity of cabling leading to such appliance to disk drive cabling, has led to unwanted coupling of electromagnetic noise.
Previously, the head preamplifier circuit has operated as a balanced input differential amplifier. The arrangement of the twisted wire pair and the balanced differential input has worked well to cancel and suppress interference appearing on each signal input line. Usually, the read preamplifier is not sensitive to a common signal on both of its differential signal inputs. Thus, the output voltage change due to a common input signal will approach zero. This common input signal or interference is known as "common mode" noise. Front end noise rejection has been measured by a common mode rejection ratio at the read preamplifier which is usually defined as a ratio of common-mode gain to differential signal gain of the preamplifier.
With the proliferation of separate MR read elements other concerns have arisen. First, the MR read element, essentially comprising a resistor formed of a very thin film of high permeability ferromagnetic conductive material, such as certain nickel iron alloys, e.g. Permalloy.TM., is highly susceptible to electrostatic damage. Therefore, it is important that the MR element be maintained at the same potential as the disk, so that if the MR element comes into electrical contact with the disk, the MR element will not be damaged by a potential difference. In practice, this requirement means that one signal path from the MR read element is grounded, and further means that the read preamplifier is operated in a single-ended configuration and loses the EMI noise cancellation advantages of the differential input arrangement. Also, as supply voltages are reduced from 12 volts, to 5 volts and more recently to 3.3 volts, there is simply insufficient voltage headroom to operate the read preamplifier in a differential input amplification mode.
EMI noise can be reduced by adding shielding to shield the front end of the disk drive. One example of shielding is provided by U.S. Pat. No. 5,270,882 to Jove et al., entitled: "Low-Voltage, Low-Power Amplifier for a Magnetoresistive Sensor", the disclosure thereof being incorporated herein by reference. The Jove et al. solution described in this prior patent was to enclose completely the head and disk assembly within a highly conductive, electrostatically shielded metallic enclosure. The enclosure essentially provided a Faraday cage which isolated the signal leads connecting the MR read elements with the preamplifier circuit from large, fast rise/fall time voltage transients. Selective shielding of sensitive components at the front end also has worked to isolate these components from unwanted pickup of magnetically coupled noise. Ferromagnetic tapes and bands have also been added to shield electrical gaps formed by a gasket between a cover and base of a disk drive. However, these prior techniques add complexity and expense to the drive. Thus, a hitherto unsolved need has remained for a simpler, less expensive way to reduce electromagnetic noise pickup and interference at the front end of a read channel of a magnetic storage device employing a single-ended preamplifier.